Accurate clock calibration is used for optimal high-speed I/O (input-output) performance. For example, clock calibration can be used for calibrating duty-cycle of a clock signal, I/Q quadrature adjustment, and clock phase calibration. Current clock quality sensors are primarily based on analog comparators and/or passive devices (e.g., capacitors, resistors, etc). These passive devices take up significant area and do not scale well over process nodes. The area overhead of analog circuits (e.g., analog comparators) and/or passive devices also limits the number and location of sensors, which limits calibration coverage.